1. Field of the Invention
The present invention relates to a CDMA base station and a method of performing uplink transmission control (TPC: Transmission Power Control) to control uplink transmission power from mobile stations.
2. Description of the Related Art
Recently, as communication technologies used for mobile communications systems, much attention has been focused on CDMA (Code Division Multiple Access) communication technology, which is robust against interference and disturbance. This CDMA communication technology is a communication scheme in which on the transmitter side a user signal to be transmitted is spread by spread coding and transmitted and on the receiver side the user signal is despread using the same spread coding as the former spread coding to obtain the original user signal.
In the CDMA communication, a plurality of transmitter sides perform spread by using different spread codes that are orthogonal to each other while the receiver sides identify respective communications by selecting the corresponding spread codes used for despreading, making it possible to share the same frequency band among communications.
However, since it is difficult to maintain perfect orthogonality among all the spread codes being used, the practical situation is that the spread codes are not perfectly orthogonal to each other but have correlated components with other codes. For this reason, these correlated components act as interference components against individual communications, causing degradation of communication quality. Since interference components are caused in this way, the interference components are compounded with the number of communications. Accordingly, if the transmission powers of all mobile stations are set at a fixed level, the radio waves from mobile stations near the base station are too strong, so that the signals from mobile stations remote from the base station are greatly subjected to interferences thereby, causing the so-called near-far problem. To address this problem, uplink transmission power control has been performed for adjusting the transmission powers of uplink signals from mobile stations to base station to appropriate values.
FIG. 1 shows a configuration of a conventional CDMA base station that performs such uplink transmission power control. As shown in FIG. 1, the conventional CDMA base station comprises packet communication circuit 1, a plurality of CDMA modulating and demodulating circuits 32, RF converter 3, multiplex circuit 4, and antenna 5,
Each CDMA modulating and demodulating circuit 32 comprises demodulator 6, modulator 7, separating circuit 8, data decoder 9, signal power estimation circuit 10, noise estimation circuit 11, SIR estimation circuit 12, TPC multiplex circuit 15, scheduling signal multiplex circuit 16 and data encoder 17.
RF converter 3 converts an uplink signal in the RF band that is input via antenna 5, into digital signal R1 in the baseband, and supplies it to demodulator 6 in each CDMA modulating and demodulating circuit 2. It also converts code-multiplexed signal T1 from multiplex circuit 4 into the signal in the RF band and outputs the resultant signal to antenna 5.
Multiplex circuit 4 performs code-multiplexing of each baseband signal T2 transmitted from modulator 7 in each CDMA modulating and demodulating circuit 32 and outputs the resultant signal T1 to RF converter 3.
Demodulator 6 performs pass-search, rake synthesis and despread on digital signal R1 from RF converter 3, and outputs uplink separate CH (R2) and packet CH (PKT1) to separating circuit 8 and packet communication circuit 1, respectively.
Transmitted from a mobile station to the CDMA base station are two uplink signals: separate uplink CH (channel) and packet CH (channel) having the configurations as illustrated in FIG. 2. FIG. 2 illustrates the configuration of the uplink signals in a W-CDMA (Wide band-CDMA) scheme.
As shown in FIG. 2, separate CH is composed of a pilot signal (Pilot), a packet request signal (RQ) and a data section, while packet CH is composed of packet data alone. Packet CH is used only for high-speed packet communication. The CHs shown in FIG. 2 all have slot configurations, and transmission power control over uplink separate CH and packet CH is also made in slot units. Transmitted from the base station to the mobile stations is the downlink separate CH shown in FIG. 2. This CH is composed of a pilot signal, a scheduling signal (SC) and a data section.
Separating circuit 8 separates the blocks of the uplink separate CH (R2) from demodulator 6 and outputs data block ULD1 to data decoder 9, pilot signal P1 to signal power estimation circuit 10 and noise estimation circuit 11, request signal RQ1 indicative of the packet transmission request from the mobile station to packet communication circuit 1.
Data decoder 9 performs error correction decoding on data block ULD1 supplied from separating circuit 8, in a predetermined manner and outputs the result ULD2 to an external data processor.
Signal power estimation circuit 10, based on pilot signal P1 supplied from separating circuit 8, estimates the signal power of the uplink signal from the mobile station and outputs the estimated uplink signal power S1 to SIR estimation circuit 12.
Noise estimation circuit 11, based on pilot signal P1 supplied from separating circuit 8, estimates the noise power of the uplink signal from the mobile station and outputs the estimated noise power N1 to SIR estimation circuit 12.
SIR estimation circuit 12 estimates a signal to interference power ratio (SIR) from uplink power signal S1 from signal power estimation circuit 10 and noise power N1 supplied from noise estimation circuit 11, and compares the result of estimation with a predetermined target SIR and outputs the result of comparison to TPC multiplex circuit 15 as TPC command TPC1. If the signal to interference ratio obtained from SIR estimation circuit 12 is greater than the target SIR, the TPC command is “0”, and If the signal to interference ratio obtained from SIR estimation circuit 12 is less than the target SIR, the TPC command is “1”. That is, TPC command TPC1 carrying “0” acts as a command to direct the mobile station to reduce the transmission power and TPC command TPC1 carrying “1” acts as a command to direct the mobile station to increase the transmission power.
Upon receipt of request signal RQ1 from separating circuit 8, packet communication circuit 1 determines whether the request should be accepted or not, based on a predetermined algorithm. Packet communication circuit 1 then outputs the result as scheduling information SC1 to scheduling signal multiplex circuit 16. Further, packet communication circuit 1 performs error correction decoding on high-speed packet signal PKT1 from the mobile station with a predetermined scheme and outputs the result PKT2 to the external data processor.
Data encoder 17 encodes data DLD1 for the mobile station, supplied from the external data processor, with a predetermined scheme and outputs the result as encoded data DLD2 to scheduling signal multiplex circuit 16.
Scheduling signal multiplex circuit 16 multiplexes scheduling information SC1 supplied from the packet communication circuit with encoded data DLD2 supplied from data encoder 17, and outputs the result DLD3 to TPC multiplex circuit 15.
TPC multiplex circuit 15 multiplexes TPC command TPC1 supplied from SIR estimation circuit 12 with DLD3 supplied from scheduling signal multiplex circuit 16 and outputs the result as data DLD4 to modulator 7.
Modulator 7 modulates data DLD4 supplied from TPC multiplex circuit 15 with a predetermined scheme and outputs the result as base band signal T2 to multiplex circuit 4.
As described above, in the conventional closed loop transmission power control performed in the CDMA base station, the increase or decrease in the transmission power of the mobile station was controlled by comparing the received SIR with a predetermined threshold. However, since SIR is a signal interference power ratio, if the amount of interference increases, then SIR is lowered, so that the base station performs control to direct the mobile station to increase the transmission power. This control is performed on all the mobile stations, tending to increase the amount of interference in the entire cell.
In order to remedy such deficiencies, a variety of closed-loop uplink transmission power control methods have been proposed (see JP11-275639A and JP2000-244391A, for example). However, none of these conventional uplink transmission power control methods do not take into consideration the case where there exists a mobile station(s) in the cell that performs uplink high-speed packet communication. Accordingly, the use of the conventional uplink transmission power control method renders the above deficiencies more conspicuous when there exist a mobile station(s) in the cell that performs uplink high-speed packet communication. When high-speed packets are transmitted from a mobile station, the transmission usually needs to be performed with a higher power than usual in order to secure transmission quality. This in turn leads to the increase in the amount of interference for other mobile stations. Therefore, when closed loop transmission power control is being performed in the uplink communication between the base station and a mobile station (from the mobile station to the base station), the transmission powers of other mobile stations are also increased in order to secure transmission quality. As a result, the amount of interference is also increased for the mobile station that is transmitting high-speed packets, further increasing the transmission power. This results in the increase in the transmission power, which causes a problem of increasing the amount of interference in the entire cell.
Since in the CDMA communication system the number of mobile stations that can be accommodated is determined by the amount of interference in the cell, the increase in the amount of interference in the entire cell leads to the decrease in the number of mobile stations that can be accommodated. To avoid this, it is common to previously determine a threshold of an amount of interference for the entire cell with which a predetermined number of mobile stations can perform communications, and perform high-speed packet communications within the threshold for the amount of interference. However, as discussed above, the presence of high-speed packets increases the amount of interference in the entire cell. In order not to reduce the number of mobile stations that can be accommodated even in that case, provision needs to be provided to lower the threshold for the amount of interference and to avoid reduction in the number of mobile stations that can be accommodated even with the slight increase in the amount of interference. However, the lowering the threshold of the amount of interference presents the problem of lowering the efficiency of use rate of communications in a cell.